The invention relates to a sensor device for detecting the turbidity of wash liquor with an optical transmitter and an optical receiver, wherein the wash liquor to be tested flows between the transmitter and the receiver. The sensor device comprises an electronic evaluation device which is designed to use the optical transmitter and the receiver to carry out a reference measurement in order to determine a reference measured value, use the optical transmitter and the receiver to carry out a turbidity measurement on the wash liquor to be measured in order to determine a test measured value, relate the reference measured value to the test measured value, and determine a characteristic value for the determined turbidity of the wash liquor therefrom and emit said characteristic value as a measurement signal.
The invention also relates to a method for detecting the turbidity of wash liquor, in particular in a domestic dishwasher, in which the wash liquor to be tested flows between a transmitter and a receiver, wherein a reference measurement is carried out in order to determine a reference measured value, a turbidity measurement on the wash liquor to be measured is carried out in order to determine a test measured value, the reference measured value is related to the test measured value, and a characteristic value for the determined turbidity of the wash liquor is determined therefrom and said characteristic value is emitted as a measurement signal.
The use of such sensor devices, also called “turbidity sensors” for short, is advantageous when operating washing machines, dishwashers and the like. By determining the turbidity of the wash or rinse liquor it is possible with repeated wash or rinse cycles to determine when this liquor is clean enough and a rinse cycle can therefore be ended. As a result it is possible to adjust the number of wash or rinse cycles or the duration of individual subprogram steps to the actual conditions with more or less heavily contaminated laundry or items to be washed and not set the rinse cycles or sub-program steps to the highest level of contamination allowed independently of the level of contamination of the wash liquor. A turbidity sensor thus contributes to a significant reduction in the amount of wash or rinse liquor required. At the same time the addition of detergent can be matched to the actual degree of contamination measured by the turbidity sensor. This means that it is also possible to reduce cleaning substances.
Turbidity Sensors are Basically Known.
A generic sensor device for detecting the turbidity of wash liquor with an optical transmitter and an optical receiver, wherein the wash liquor to be tested flows between the transmitter and receiver, is known from DE 44 03 418 A1. The optical conditions in the measuring room are firstly determined herein by carrying out a reference measurement before the actual turbidity measurement. This means the degree of contamination of the turbidity sensor and/or the possible degree of contamination of the reference liquid is determined. This measured value defines the contamination in the measuring room and is defined as the base value. The actual measured value that then takes place (on the basis of the measurement of the contaminated liquid for testing) is related to the reference value. The resulting difference is processed further as the measured signal as the relative contamination or turbidity.
To carry out the reference and/or test measurement a gradually increasing voltage is applied to the optical sensor by a digital-to-analog converter in order to generate a gradually increasing brightness in the sensor. When a sufficiently bright signal is detected the receiver emits an electrical signal to an evaluation device which then ends generation of the voltage for the transmitter and generates the measurement signal. At the instant at which the receiver detects a signal that is sufficiently bright to penetrate turbid liquid additional voltage levels are ended for the optical transmitter by generating an electrical signal at the output of the receiver. The counter count reached in the evaluation device is “frozen” and is used as a gauge of the measured turbidity.
DE 101 11 006 A1 discloses a method for adjusting and correcting a turbidity sensor to changing conditions at the measuring site, calibration or reference values being determined as the wash program is running in order to adjust the sensor and correct the turbidity sensor in the case of turbidity measurements on the wash liquor. A plurality of calibration measurements is carried out within one wash program in this case, at instants at which it is highly probable that there is clear water at the measuring site of the turbidity sensor in each case. The most optimal reference value is used for turbidity value correction, and this is obtained by averaging a plurality of reference values that were determined in a plurality of wash program cycles.
DE 103 56 279 A1 describes a sensor switch for detecting a level of turbidity in a liquid, for example the wash liquid of a dishwasher. However, in this case it is only possible to detect certain levels of turbidity that are fixed by the configuration of the electronic evaluation device. It is not possible to determine an absolute level of turbidity.
DE 101 19 932 A1 describes a transmission sensor with first and second measured sections. A transmitter emits electromagnetic radiation in both measured sections. The first measured section is associated with a first receiver and the second measured section is associated with a second receiver. A measured value calibration is carried out in which a first calibration value that correlates with the intensity of the radiation transmitted by the first measured section and a second calibration value that correlates with the intensity of the radiation transmitted by the second measured section are determined. Normalized measured values are used when determining a turbidity value that correlates with the turbidity of the fluid. A first measured value that correlates with the intensity of the radiation transmitted by the first measured section and a second measured value that correlates with the intensity of the radiation transmitted by the second measured section are firstly determined for this purpose. A first normalized measured value is then formed by the quotient from the first measured value and first calibration value and a second normalized measured value is formed by the quotient from the second measured value and second calibration value. The turbidity value is determined using these normalized measured values. The state of the fluid during the measured value calibration is defined as the reference state, so the determined turbidity value indicates the deviation from this reference state. The purpose of this procedure is to eliminate the effects of dirt depositing on the transmitter and/or receivers. Aging phenomena in the transmitter and/or receivers are also neutralized. Power fluctuations in the transmitter and receivers used, which can occur during production, are also neutralized.
FIG. 1 shows a schematic diagram of an electrical equivalent circuit diagram of a sensor device 1, used, for example, in domestic dishwashers. The sensor device 1 comprises a turbidity sensor 2 which is also called an “aqua sensor”. The turbidity sensor 2 comprises an optical transmitter 3, coupled to a supply voltage Vcc, and a receiver 4, in the form of a phototransistor, coupled to the optical transmitter 3. As is known, the luminous intensity emitted by the transmitter 3 depends on the current that flows through it. For this purpose a circuit 5 coupled to the transmitter 3 is provided which can be controlled via an output 12 of a control and evaluation device 11. The transmitter 3 is controlled by means of pulse-width modulation in that a transistor 7 in the circuit 5 connects the transmitter 3 to a reference potential via a resistor 6 according to its control. Depending on the pulse-duty factor set, an average current is produced through the transmitter 3, resistor 6 and conductive transistor 7 in the direction of the reference potential. The receiver 4, designed as a phototransistor, is coupled by its emitter via a circuit block 10, configured as an anti-aliasing filter, to an input 13 of the control and evaluation device 11. A pulsed current is applied to the input of the circuit block 10 due to the transmitter 3 being controlled with a pulse-width modulated signal. The pulsed photocurrent is converted into direct voltage Ua by the circuit block 10. FIG. 2 shows one possible embodiment of the circuit block 10. A resistor 14 converts the current applied to the input into a voltage Ue. The charge-coupled accumulators 15 and 17 and the resistor 16 are used for low-pass filtering the input signal. The output voltage Ua, which can be supplied for further evaluation to the control and evaluation device 11 via its input 13, can be tapped at the charge-coupled accumulator 17.
Conventionally both the receiver 4 and the transmitter 3 of the sensor device 1 have different manufacturing tolerances, for which reason it is necessary to calibrate the sensor device. For this purpose the sensor device is firstly operated in defined conditions to determine a reference measured value. This conventionally takes place with clear wash liquor or in the absence of wash liquor.
If the sensor device 1 described in connection with FIG. 1 is operated with different pulse-duty factors, an output voltage Ua that is dependent on the pulse-duty factor is produced, and this is passed to the control and evaluation device 11 for further processing. FIG. 3 shows the connection between the pulse/pause ratio PWM (pulse-duty factor) and voltage UAD (=Ua), which has already been A/D converted in the figure at hand. The figure is based on the assumption that an analog-to-digital converter with an 8-bit width is used. The pulse-duty factor is likewise shown in digital form.
To calibrate the sensor device, the sensor, starting from a pulse-duty factor 0, is operated with an increasing pulse-duty factor and, at the same time, the output voltage Ua is monitored by the control and evaluation device 11. As soon as the voltage Ua, which matches the measure of turbidity, is within a predefined calibration window (cf. reference character 100 in FIG. 3), the calibration procedure is terminated and the pulse-duty factor present in the case of the determined voltage UAD (=Ua) is stored for the additional measuring processes. This procedure can clearly be seen in FIG. 3 in which measured curve MK1 depicts the characteristic of the voltage or turbidity as a function of the pulse-duty factor during calibration of the sensor device. As may easily be seen from the figure, measured curve MK1 has a linear region and a saturation region. Measured curve MK1 crosses the calibration window 100 in its saturation region. As may easily be seen, calibration can be terminated at a voltage between 160 and 190 digits (cf. reference characters 103 and 104). The pulse-duty factors 101, 102 associated with voltages 103, 104 are plotted accordingly.
Pulse-duty factors that differ from each other can therefore result depending on where voltage Ua comes to rest within the voltage difference 105, for which reason calibration has a certain inaccuracy.
Additional measured curves MK2, MK3, MK4, MK5, MK6, MK7 and MK8 are plotted in FIG. 3, and these exhibit increasing turbidity in their linear regions. The turbidity of the measured wash liquor increases as the gradient decreases here.
Determination in the described manner of a reference value in the case of clear or no wash liquor therefore results in the sensor device being operated with a relatively low pulse-duty factor for the further measurements with contaminated wash liquor.